It is here particularly relevant to consider the shrimp fishing in the arctic, where the operation depth is 500-1,000 meters or more. The applied trawl bags should be held open with the use of weight bodies at the lower side of the bag mouth and buoyancy bodies at the top side of the mouth; however, due to the large working depth, there are special demands on the buoyancy bodies because they are subjected to an excessive outer pressure.
Conventionally, these bodies are made as air-filled ball shells of plastic, but it is proved difficult to produce reliable buoyancy bodies of this type because, from resisting the high pressure, the ball shells must be so thick that reasonably small buoyancy balls will exhibit only a limited buoyancy while, with the use of large buoyancy balls, a considerable resistance against the trawl being drawn through the water is created.
Moreover, the buoyancy balls are normally required to have a central through-passage for a rope, and, while the ball shell shape is well suited to resist high outer pressures, it is so very difficult to produce the ball shells inexpensively, which are nevertheless effective and durable with buoyancy balls having such a passage. Buoyancy balls may well initially resist a high pressure, but they become weakened by the considerable pressure variations to which they are exposed by the repeated lowerings and raisings to and from the large depths.